Pierluigi Paganini November 24, 2020

Microsoft released an out-of-band update for Windows to address authentication flaws related to a recently patched Kerberos vulnerability.

Microsoft released an out-of-band update to address authentication issues in Windows related to a recently patched Kerberos vulnerability tracked as CVE-2020-17049.

“An out-of-band optional update is now available on the Microsoft Update Catalog to address a known issue affecting Kerberos authentication. As part of this issue, ticket renewal and other tasks, such as scheduled tasks and clustering, might fail.” reads the advisory published by the IT giant. “This issue only affects Windows Servers, and Windows 10 devices and applications in enterprise environments.”

The flaw is related to the PerformTicketSignature registry subkey value in CVE-2020-17049, which is a security vulnerability bypass bug in Kerberos Key Distribution Center (KDC) that Microsoft addressed with the release of November 2020 Patch Tuesday security updates.

“A security feature bypass vulnerability exists in the way Key Distribution Center (KDC) determines if a service ticket can be used for delegation via Kerberos Constrained Delegation (KCD).” reads the advisory published by Microsoft.

“To exploit the vulnerability, a compromised service that is configured to use KCD could tamper with a service ticket that is not valid for delegation to force the KDC to accept it.

Last week, the company disclosed a series of flaws that could occur on writable and read-only domain controllers (DC) such as:

• Kerberos service tickets and ticket-granting tickets (TGT) might not renew for non-Windows Kerberos clients when PerformTicketSignature is set to 1 (the default).
• Service for User (S4U) scenarios, such as scheduled tasks, clustering, and services for line-of-business applications, might fail for all clients when PerformTicketSignature is set to 0.
• S4UProxy delegation fails during ticket referral in cross-domain scenarios if DCs in intermediate domains are inconsistently updated and PerformTicketSignature is set to 1.

Microsoft recommends installing this optional update on Domain Controllers only if their customers are affected, the company warns of some problems related to the Microsoft Input Method Editor (IME) for Japanese or Chinese languages.

Pierluigi Paganini

(SecurityAffairs – hacking, Microsoft)

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